The present invention is directed to a thermal printing method wherein, for a cyclical drive of heating elements of a thermal printing head, the temperature of each and every individual heating element is first measured in every drive cycle before the heating element intended for printing is charged with a current pulse, the energy of this current pulse being controlled as a function of the measured temperature.
In a prior art thermal printing method disclosed in German published application No. 1 964 389, a drive of the heating elements of a thermal printing head occurs such that the momentary temperature of each and every individual heating element intended for printing is measured individually and successively with a temperature sensor individually allocated thereto. The heating element is charged with a current pulse immediately thereafter, the current magnitude of this current pulse being controlled as a function of the measured temperature by an amplifier following the temperature sensor. This is intended to reduce the influences of the ambient temperature and of the temperature of the printing head on the printing quality, this latter temperature of the printing head being dependent on the drive of the heating elements that was previously applied to the heating elements. Since the respective heating elements intended for printing are driven in individual succession, the printing time required for printing a point grid corresponding to the arrangement of the heating elements is relatively great.
In a thermal printing method disclosed by European Pat. No. EP-A 0 112 474, a simultaneous, temperature-dependent current charging of heating elements of a thermal printing head is provided. The momentary, actual temperature conditions at the individual heating elements are thereby left out of consideration since the temperature measurement and, thus, the current charging occurs globally for all heating elements.
Finally, for identifying malfunctioning heating elements of a thermal printing head, European Pat. No. EP-A-0 174 751 discloses that the electrical resistances thereof be measured by charging the heating elements with a test current individually and in succession. The measured values of resistance are compared to corresponding reference values stored in a memory, whereupon the appertaining heating element is recognized as having malfunctioned for a deviation that exceeds a prescribed level.